1. Field of the invention
The present invention relates to a memory device and a method for manufacturing the same, and more particularly to a new type of transistor realizing both a fin transistor and a step-gated asymmetry transistor in a unit cell and a method for manufacturing the same.
2. Description of the Prior Art
As semiconductor devices are highly integrated, the design rule of high-integration MOSFET devices is drastically reduced to sub-100 nm level and the corresponding channel length and width of transistors are remarkably decreased. Due to the current trend towards a short channel structure, the threshold voltage cannot be independent of the channel width any longer. Therefore, it is generally accepted that, when realizing the target threshold voltage of a transistor requested in a specific device, the conventional two-dimensional planar structure has reached its limit.
In order to overcome this problem, three-dimensional transistors are actively studied these days in logic devices. In particular, fin transistors having triple channels are considered as the core technology requested in the next-generation logic devices. The fin transistors are designed to have channels formed on three surfaces by realizing an active having a three-dimensional structure, so that they have excellent current driving properties as the amount of current increases instantly. In addition, they have very excellent on-off properties and can realize high-speed devices. Due to decreased back bias dependency, they can have desired device properties in low voltage. For these properties, highly-integrated circuits can be realized.
However, three-dimensionally structured transistors have a problem in that they cannot secure enough data retention time, which is the most important property cells must have. This is because the source of junction leakage current is expected to become much bigger due to the triple channels formed in the narrow active region. Therefore, it is requested to secure enough data retention time when a transistor having such a structure is used in a memory device (for example, DRAM).
In summary, although conventional fin transistors have excellent on-off properties and high driving current properties and the possibility of using them as logic devices is actively studied, they are not yet applicable to memory devices due to poor data retention time.